"I was a gift that contained all the world's evils. I was opened out of curiosity and could never be closed again. Only one thing remained inside me when it was over. What am I?"

Answer: Pandora's Box. In Greek mythology Pandora was given a box (originally a jar) and told never to open it. Curiosity overwhelmed her and when she opened it all the world's evils flew out. She managed to close it before the last thing escaped — Hope.